1. Field of the Invention
This invention relates to a temperature sensing circuit, and more particularly to a temperature sensing circuit fabricated on a thin-film transistor substrate.
2. Description of the Related Art
In modern days, video display devices play an important role in our daily life. Information and communication massages are transmitted and then displayed in those devices. Generally, display devices are classified into luminous types and non-luminous types. Luminous type display devices are cathode ray tube (CRT) and light emitting diode (LED), while non-luminous type displays include liquid crystal display (LCD) and the likes.
The LCD displays offer the advantages of compact volume and power saving compared with the conventional CRT displays. A liquid crystal display device capable of performing the color display by making use of a display element such as liquid crystal, and combining the light source and a color filter has been known. For example, U.S. Pat. No. 6,513,236 by Tsukamoto disclosed such a LCD package structure (the entire disclosure of which is herein incorporated by reference). A thin film transistor controls the liquid crystal display device in which a picture element to perform one color display is constituted by combining three primary colors of red (R), green (G) and blue (B). A large number of the picture elements is arranged in the display region: the signal line and the scanning line are arranged in the matrix to drive the liquid crystal; the pixel electrode is arranged in the region demarcated by the signal line and the scanning line; switching to the pixel electrodes is performed by the thin film transistor; the electrical field is applied to the liquid crystal corresponding to each pixel; and the transmittance ratio of the liquid crystal is changed to switch the display/non-display.
The active matrix LCD (AMLCD) uses a thin-film transistor (TFT) substrate to form image pixels and to provide driving current. Therefore, it fulfills the requirements of being lightweight/thin/small in volume and reducing the production cost. Referring now to FIG. 1, it shows a schematic diagram of a conventional AMLCD pixel cell structure.
The TFT cell includes a transistor 102 to drive an LCD device 106. The transistor 102 has a gate connected to a scan line, a source connected to a data line, and a drain connected to the anode of the LCD device 106, which has a cathode further connected to the ground. When the scan line goes high, the transistor 102 turns on; thereby the data line voltage VDATA is input into the LCD device 106 to turn on the pixel.
The characteristics of a TFT LCD, such as response time and contrast, are easily affected by temperature variation. A temperature sensing and control circuit is usually incorporated on the TFT LCD display panel to compensate this effect. Conventionally, the temperature sensing circuit is made of a series of PN junctions, such as that disclosed in U.S. Pat. No. 5,366,943 by Kelly et al. (the entire disclosure of which is herein incorporated by reference). However, in the materials currently employed for TFT substrate such as amorphous silicon (α-Si) or polysilicon, no PN junction exists in the TFT substrate. Therefore, there is a need for temperature sensing circuit on the TFT substrate. There is also a need for a new temperature sensing circuit which can be fabricated without the PN junction. Further, there is a need to effectively control the temperature of the TFT substrate.